Tarnish remover



United States Patent TARNISH REMOVER Harold Robert Treece, Leicester, England, assignor of one-half to The Design and Research Centre for the Gold, Silver, and Jewelry Industries, London, and onehalf to I. Goddard & Sons Limited, Leicester, England No Drawing. Application October 19, 1954, Serial No. 463,315

Claims priority, application Great Britain October 22, 1953 7 Claims. Cl. 204-444 This invention is concerned with improvements in or relating to the removal of tarnish from silver articles.

A process has previously been developed for removing tarnish from silver articles in which tarnished silver articles are treated with an aqueous solution of an acid which is stronger than hydrosulphuric acid but is not such as will, in aqueous solution, attack silver to any material extent and a soluble compound capable of forming a soluble complex with silver ions, which complex gives rise to substantially no silver ions in aqueous solution, such as thiourea or thiosemicarbazide. Whilst it is found that such process is very satisfactory with articles of silver and silver alloys and with silver-plated articles in good condition, a disadvantage frequently arises when the process is applied to badly worn silver-plated articles, that is articles in which some of the basic metal, that is the metal upon Which the silver or silver alloy has been plated, has become exposed. It has been found that when the detarnishing process of the said prior application is applied to badly worn plated articles, in many instances the exposed basic metal becomes discoloured; such discolouration may be marked, particularly where the articles are left in the detarnishing solution for any length of time.

The effect only occurs when the carrier metal is more electropositive than silver, and since copper and nickel silver (an alloy of copper, zinc and nickel) are commonly used as the basic metal, the phenomenon is quite frequent. Whilst we do not wish to be limited by a theoretical explanation, we believe that the effect is due to the formation of a cell with the exposed basic metal as one pole and the silver :as the other pole. As the exposed basic metal is more electropositive than the silver, in operation, negatively charged ions are discharged at the basic metal and thus bring about the discolouration above referred to, whilst hydrogen ions are discharged at the silver with evolution of hydrogen.

The previously developed process has also another disadvantage, in that where the articles to be cleaned include other metals, as for example, silver-handled knives having steel blades or articles of 9-carat gold and silver, un desirable elfects may be obtained. Thus the steel blade of a silver handled knife will be attacked by the process of the said prior application whilst with articles composed of silver and 9-carat gold, silvering of the gold may take place. In general terms, it may be said that disadvantages are likely to be met when the silver to be cleaned is associated with another metal which we shall call for convenience a secondary metal.

A general object of the present invention is to provide a means whereby the above-mentioned disadvantages arising from the application of the previously developed detarnishing process can be reduced or removed.

We have found thatthe disadvantages mentioned. above can be reduced or avoided by providing some means in the detarnishing operation whereby the operation of a cell between the silver and any secondary metal present is prevented; this effect is obtained by providing means during the detarnishing process for giving the secondary metal negative electrochemical characteristics.

The presentinvention therefore comprises an agent for removing tarnish from silver, silver alloy and silver-plated articles comprising an aqueous solution of an acid which is stronger than hydrosulphuric acid but is not such as will in aqueous solution attack silver to any material extent and a soluble compound capable of forming a soluble complex with silver ions which complex gives rise to substantially no silver ions in aqueous solution, in combination with means for giving the silver and any secondary metal associated therewith negative electrochemical characteristics during the detarnishing operation.

By the use of the present process for example we are able to detarnish worn silver plate without any substantial discolouration of the worn areas. Furthermore the need to sort out articles containing metals other than silver may be avoided which obviously is of importance particularly Where large amounts of silver have to be cleaned as in hotels and restaurants.

We will now describe, by way of example only, two embodiments of a process of detarnishing according to the invention. In one embodimentwe carry out the detarnishing process in the presence of a metal which is more electropositive than the silver and any secondary metal associated therewith, such metal being in electrical contact with the silver article to form a cell. For example where copper is used as the carrier metal in silver plate, metals which are more electropositive in the electrochemical series than copper can be used. It will be apparent that certain of such elements will be quite unsuitable as they will be too vigorously attacked by water or by the acid of the detarnishing solution. The metals which may be used can therefore be characterised as being those which are more electropositive than the carrier metal and which are not attacked to any material extent by the detarnishing solution. Suitable elements where copper is the secondarymetal are, forexample, aluminium, and nickel and where the-secondary metalis nickel, as in nickel silver, or iron, aluminum is satisfactory. Such metals may be present in the detarnishingv process in any convenient form; thus the container in which the detarnishing process is carried out may be formed wholly or partly from them, or they may be introduced into the detarnishing solution, before introduction of the silver articles to be cleaned, for example in the form of a plate, rod, turnings or filings; such metals in all cases being in electrical contact with the silver articles. The eifect of incorporating such metals is that being more electropositive than the basic metal, they attract the negatively charged ions and thus prevent discolouration of the basic metal.

In one convenient method of operation, the detarnishing liquid is placed in a container of suitable material, for example polyvinyl chloride or polyethylene, the silver articles to be cleaned are placed ni an aluminium basket and the basket dipped in the solution. After removal from the detarnishing solution, the basket may be dipped in plain water in order to rinse the silver.

In another method of operation the detarnishing solution is electrolysed, the article being treated forming the cathode. The container of the detarnishing solution may, for example, be formed from an electrical conductor and be connected as the anode; alternatively the anode may simply be introduced into the solution as a plate, rod or the like. In this method of operation if the electrolysis is carried on too long, in general past the point when detarnishing is complete, a deposit may form on thesilver, which is not however coherent and may be readily removed.

It is to be noted that one effect of the process according to the invention is to accelerate the detarnishing action of the said previously developed process and it is therefore advantageous to apply the process according to the invention to the detarnishing of silver of all kinds whether a secondary metal is present or not. We further find that silver articles associated with a secondary metal can be rapidly and effectively detarnished by the hereindescribed process without substantial discolouration of the secondary metal occurring even when such articles are left in the detarnishing solution for some length of time.

The complex-forming compounds which may be used in the process according to the invention are, preferably, thiourea or thios'emicarbazide. Preferably the detarnishing solution contains at least 1% by weight of said complex-forming compound and preferably a solution containing about 5% of said complex-forming compound is employed.

Various acids may be employed, examples of which are hydrochloric 'acid, sulphuric acid, citric acid, chloracetic acid, aminosulphonic acid, and malic acid. In the modification according to the invention wherein cell-formation between the silver and the secondary metal is prevented by the presence of another metal which is more electropositive than the secondary metal the acid employed must be capable of dissolving said other metal. Thus where aluminium is employed to prevent cell formation hydrochloric acid is conveniently used but sulphuric acid by itself is unsuitable; the rate of reaction of hydrochloric acid with aluminium is however inconveniently fast so that mixtures of sulphuric acid or aminosulphonic acid with hydrochloric acid or chloracetic acid are conveniently used, the respective proportions of each acid in which are balanced to provide a satisfactory rate of reaction. A suitable acid mixture contains sulphuric acid and hydrochloric acid in the ratio of 9:1 by weight. The actual acid concentration of the detarnishing solution will depend on the strength of the acid employed and the rate of detarnishing desired; in the case of strong acids a suitable concentration is from 0.03% to 6.0%. Preferably the acid concentration is at least 0.15%.

A further advantage of this invention is that the loss in weight (by dissolution of the silver) of silver articles treated according to the said prior process is reduced and thus for example in tests we have carried out, a number of silver-plated forks have been allowed to remain in the detarnishing solution in contact with aluminium for several months without harmful effect on the silver; were the aluminium not present etching of the silver would have taken place after a day or two.

It will be apparent to those skilled in the art that the two embodiments described above do not represent the only ways in which the process of the present invention may be put into effect and we do not wish to be limited to the hereinbefore described embodiments, which are given solely by way of illustration, without any limitation thereto being intended.

In order that the invention may be well understood the following examples are given only as illustrations.

Example 1 A detarnishing solution was made up containing:

Percent by weight Thiourea 5 Hydrochloric acid 1 Water To 100 A small quantity of a wetting agent (oleyl/cetyl alcohol/ethylene oxide condensate) compatible with acid was added. This solution was placed in a glass container together with a small plate of aluminium. A heavily tarnished electroplated nickel silver fork, the prongs of which were badly worn, was placed in the solution in contact with the aluminium plate. Detarnishing was very rapid and even though the fork was left in the solution for 30 minutes, no discolouration of the exposed nickel silver took place.

The experiment was repeated without the use of the aluminium plate; the detarnishing was not so rapid. After a few minutes contact with the solution, the exposed nickel silver was badly discoloured.

Example 2 The detarnishing solution used in Example 1 was placed in a glass container, together with a carbon electrode connected to a 4.5 volt D. C. supply as the anode. A heavily tarnished electroplated nickel silver fork, the prongs of which were so badly worn that some of the nickel silver was exposed, was then connected to the same supply, as the cathode. The fork was placed in the detarnishing solution and the current switched on. Detarnishing was very rapid and no discolouration of the exposed nickel silver took place. If the application of black deposit may be formed which is not however coherent and may be readily removed by simple polishing.

Example 3 A detarnishing solution was made up containing:

Percent Thiosemicarbazide 4 Hydrochloric acid 2 Wetting agent 0.5 Water Up to The solution was placed in a polythene container together with an aluminium electrode connected to the positive pole of a 4.5 volt D. C. supply. A heavily tarnished electro-plated nickel silver fork, the prongs of which were so badly worn that some of the nickel silver was exposed, was then connected to the negative pole of the same D. C. supply and was then placed in the detarnishing solution, and the current was switched on. Detarnishing was rapid, and even though the fork was left in the solution for over fifteen minutes, no discolouration of the exposed nickel silver took place.

Example 4 A detarnishing solution was made up as follows:

Percent Thiourea 5 Sulphuric acid 0.9 Hydrochloric acid 0.1 Wetting agent 1 Water Up to 100 The solution was poured into a container fabricated from rigid polyvinyl chloride in which was nesting a woven wire aluminium basket. The basket was connected to the positive pole of a 4.5 volt D. C. supply, and a very heavily tarnished solid silver antique cup (which had been rescued from a fire by the local museum authorities) was connected to the negative pole of the same D. C. supply and lowered into the detarnishing solution for a minute or so. After this treatment, the very heavy deposit of tarnish was easily removed by light brushing with a soft-bristled brush.

Example 5 A two gallon batch of detarnishing solution made up as in Example 4 was poured into a specially designed container which was fabricated from rigid polyvinyl chloride.

A batch of tarnished cutlery comprising:

30 E. P. N. S. (electro-plated nickel silver dessert forks with worn prongs 30 E. P. N. S. dessert spoons with worn bowls 30 dessert knives with E. P. N. S. handles and stainless steel blades 30 E. P. N. S. teaspoons was loaded into an aluminium woven wire basket designed to fit into the outer polyvinyl chloride container holding the detarnishing solution. The basket of tarnished cutlery was then lowered into the detarnishing solution and after a few seconds immersion removed and well rinsed in running water. After the cutlery had been dried on a soft cloth a close examination showed that the silver-plated articles had been thoroughly detarnished, and there was no discolouration of those items where the basic metal was exposed; nor was there any discolouration of the stainless steel knife blades.

Example 6 A detarnishing solution was made up having the following composition:

Percent by weight Thiourea Sulphuric acid 0.9 Chloracetic acid -1 0.1 Wetting agent 0.5 Water Up to 100 The solution was poured into an aluminium container and a very heavily tarnished silver bracelet of intricate design was introduced into the detarnishing solution so that metallic contact was made with the inside of the aluminium container. The bracelet was speedily detarnished.

Example 7 A detarnishing solution was made up having the following composition:

Percent by weight Thiourea 5 Sulphuric acid 0.9 Trichloracetic acid 0.1 Wetting agent 0.5 Water Up to 100 Example 8 A detarnishing solution was made up having the following composition:

Percent by weight I'hiourea 5 Aminosulphonic acid 2 Hydrochloric acid 0.1 Wetting agent 0.5 Water Up to 100 The solution was placed in a glass container together with a small plate of aluminium. A heavily tarnished electroplated nickel silver fork, the prongs of which were badly worn, was placed in the solution in contact with the aluminium plate. Detarnishing was very rapid, and even though the fork was left in the solution for thirty minutes, no discolouration of the exposed nickel silver took place.

I claim:

1. A method of detarnishing articles comprising silver and a secondary metal in which the surfaces of said articles have exposed thereon both silver and secondary metal, without tarnishing the exposed secondary metal, comprising: immersing said articles in an aqueous solution which will not materially attack silver, said solution having dissolved therein an acid stronger than hydrosulphuric acid and at least 1% by weight of a compound selected from the group consisting of thiourea and thiosemicarbazide capable of forming a soluble complex with silver ions in aqueous solution; and imparting to said articles negative electrochemical characteristics during the de-tarnishing operation.

2. A method of detarnishing articles comprising silver and a secondary metal in which the surfaces of said articles have exposed thereon both silver and secondary metal, without tarnishing the exposed secondary metal, comprising: immersing said articles in an aqueous solution which will not materially attack silver, said solution having dissolved therein an acid stronger than hydrosulphuric acid and at least 1% by weight of a compound selected from the group consisting of thiourea and thiosemicarbazide capable of forming a soluble complex with silver ions in aqueous solution; and imparting to said articles negative electrochemical characteristics during the de-tarnishing operation by placing in said aqueous solution in contact with said articles another metal more electropositive than any metal in said articles, said acid being capable of dissolving said other metal.

3. The method of claim 2 in which said acid stronger than hydrosulphuric acid is selected from the group consisting of hydrochloric acid, sulphuric acid, citric acid, chloracetic acid, aminosulphonic acid and malic acid.

4. The method of claim 2 in which said other metal is aluminum.

5. The method of claim 4 in which said acid stronger than hydrosulphuric acid is a mixture of an acid selected from the group consisting of sulphuric acid and aminosulphonic acid with an acid selected from the group consisting of hydrochloric acid and chloracetic acid.

6. The method of claim 4 in which said aluminum comprises a basket in which said articles are placed and immersed in said aqueous solution.

7. a method of detarnishing articles comprising silver and a secondary metal in which the surfaces of said articles have exposed thereon both silver and secondary metal, without tarnishing the exposed secondary metal, comprising: immersing said articles in an aqueous solution which will not materially attack silver, said soluton having dissolved therein an acid stronger than hydrosulphuric acid and at least 1% by weight of a compound selected from the group consisting of thiourea and thiosemicarbazide capable of forming a soluble complex with silver ions in aqueous solution; and imparting to said articles negative electrochemical characteristics during the de-tarnishing operation by making said articles the cathode of an electrolysis carried out in said aqueous solution as electrolyte.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF DETARNISHING ARTICLE COMPRISING SILVER AND A SECONDARY METAL IN WHICH THE SURFACES OF SAID ARTICLES HAVE EXPOSED THEREON BOTH SILVER AND SECONDARY METAL, WITHOUT TARNISHING THE EXPOSED SECONDARY METAL COMPRISING: IMMERSING SAID ARTICLES IN AN AQUEOUS SOLUTION WHICH WILL NOT MATERIALLY ATTACK SILVER, SAID SOLUTION HAVING DISSOLVED THEREIN AN ACID STRONGER THAN HYDROSULPHURIC ACID AND AT LEAST 1% BY WEIGHT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF THIOUREA AND THIOSEMICARBAZIDE CAPABLE OF FORMING A SOLUBLE COMPLEX WITH SILVER IONS IN AQUEOUS SOLUTION; AND IMPARTING TO SAID ARTICLES NEGATIVE ELECTROCHEMICAL CHARACTERISTICS DURING THE DE-TARNISHING OPERATION. 